Fix broken unit tests

Remove some obsolete tests from BRDF test
file (these methods were moved to libtools
and there are tests there for them)

Test for get_reflectance_all was falling
over because the mocked open image didn't
have any values set for RasterCount,
RasterXSize and RasterYSize. So add mock
values in the test.
Refactor get_reflectance_all method

Simplify the get_reflectance_all method to
use the image's .ReadAsArray() method to read
all bands at once, instead of looping through
the bands and reading one at a time.

Amend the unit test, as we need to change the
mock return value accordingly.